Emerging Threats

The Scoop on Shellshock

If you're an IT professional, chances are you were rudely awoken this week to a critical vulnerability-called "Shellshock"-that is present in the bash shell, the default command-line interpreter for Unix-based systems such as Linux and Mac OS X.

Many reports are calling the vulnerability more severe than the Heartbleed vulnerability, which emerged earlier this year and left hundreds of thousands of servers worldwide open to attack. Because of the apparent danger associated with this new bug, we have pulled together a quick FAQ to help guide you through the uncertainty.

Before we jump into that, we want to reassure our customers that we have no reason to believe Trustwave products and services use Bash in a way that would allow this vulnerability to be exploited by attackers. We continue to verify, test and update our solutions to help ensure your systems remain secure. Customers with more specific questions should contact Trustwave support.

What is Shellshock?

"Shellshock" is a severe vulnerability found in the Linux command shell, Bash. The vulnerability - discovered by researchers last week and disclosed on Wednesday - allows an attacker to remotely execute arbitrary Linux commands through easily crafted environment variables.

So what does that mean? Well, since many applications use Bash "behind the scenes," there are multiple attack vectors, including implementations of OpenSSH and Apache web servers running the Common Gateway Interface (CGI), used to generate dynamic content on web pages. This vulnerability involves the way Bash handles environment variables, and it can result in malicious commands - such as ones appended to a CGI script - being sent.

So, what can I do?

Organizations should assess their infrastructure to determine whether they are affected by the bug, then they should patch as quickly as possible. Check with Linux and Unix providers for up-to-date patches. Additional recommendations from our security experts:

Deploy a web application firewall - Trustwave Web Application Firewall has command injection rules that can help detect payloads of this attack while providing a built-in measure of protection. We're also investigating the creation of rules specific to this vulnerability (CVE-2014-6271).

Intrusion detection - Trustwave UTM and intrusion detection solutions have deployed signatures to detect the most common types of activity associated with exploiting this vulnerability. More signatures will be developed and deployed as we continue our research.

Scanning - Trustwave vulnerability scanning engines are being updated to detect the Shellshock vulnerability. This is expected to be implemented soon. Following the update, our scanning engines will automatically look for this threat. Trustwave App Scanner (formerly known as Cenzic Hailstorm) is also expected to feature additional detection capabilities at that time.

Is this new bug really as bad as Heartbleed?

It's unclear. Certainly the vulnerability-which has earned the most severe score (10.0) a vulnerability can be assigned, according to NIST-has the capability to be far-reaching considering every version of Bash, up until 4.3, is impacted. Like Heartbleed, an enormous amount of devices could be vulnerable. And now with reports of proofs-of-concept and active exploits underway, the situation could get severe in a hurry. As we saw with breaches attributed to Heartbleed months after the news of that bug broke - the damage could linger for a very long time to come.

UPDATE 10/1/14: We have posted a new blog containing specific details about how Trustwave products and services can help detect and block Shellshock. Trustwave will continue to investigate this vulnerability and provide more details as they become available. If you are a Trustwave customer with more specific questions, please contact Trustwave support.

Dan Kaplan is manager of online content at Trustwave and a former IT security reporter and editor.